Anomalous Features in Surface Impedance of Y-Ba-Cu-O Thin Films: Dependence on Frequency, RF and DC Fields

Two high-quality Y-Ba-Cu-O thin films on MgO substrates have been investigated using the coplanar resonator technique at 8 and 16 GHz. Both films exhibit an anomalous decrease in their surface impedance, Zs as a function of microwave field, Hrf. In zero dc field, Hrf-dependences of Rs and Xs for both the samples are uncorrelated, and only one of the quantities, Rs or Xs, displays anomalous behavior. Here, application of relatively weak (~5 mT) dc magnetic fields, Hdc can produce a correlated decrease of Rs(Hrf) and Xs(Hrf). The dependences of Zs on Hdc in both low and high microwave power regimes were found to be non-monotonic. The frequency dependence of Rs ~ fn, 1.7<n<2.5, remained the same upon the transition from low to high microwave power ranges. The consequences of the reported findings for microwave device applications are briefly discussedComment: 4 pages, 4 figures. Submitted to ASC'2000 Conference Proceeding

Flood lava flow fields on the plains of Venus

The Magellan spacecraft has mapped nearly 99% of the surface of Venus with high resolution synthetic aperture radar (SAR) images, radiometry and altimetry. These data are used to study the characteristics and distribution of volcanic deposits on Venus, specifically the large flood-type lava flow fields, and their relationship to tectonic features. Regional stratigraphy and the nature of plains formation are also investigated. A set of fifty flood-type lava flow fields on Venus with total areas greater than 50,000 km2 (great flow fields) are analysed. These fields have typical lengths of several hundred kilometres and estimated volumes of 103 to 104 km3. The fields have been classed into five morphological types, with the basic distinction being drawn between sheet-like and digitate morphologies. The sheet flow fields may represent single massive volume limited eruptions from laterally extensive fissures, while the digitate fields were formed by the repeated eruption of individual lobate flows. Six eruptive episodes are identified in Mylitta Fluctus, which is studied in detail. Most of the great flow fields may be characterised by extremely smooth pavement-like surface textures, with some occurrences of aa. First order estimates of eruption rate of between 105 and 106 m3s-1 have been made for typical flow units within the digitate fields. Most of the great flows are associated with zones of lithospheric extension and thinning, and represent comparatively recent volcanism. However, a population of indistinct flows on the plains may represent an extended period of plains resurfacing. The great flow fields are considered to be Venusian analogues of terrestrial flood basalt provinces. A specific regional study of Rusalka Planitia and Atla Regio has indicated three main phases in the geological history of that area, including early tessera formation, widespread plains formation and the burial of tessera, and later mantle upwelling and the formation of coronae, large volcanoes and rift zones

Global Optimization of Microwave Filters Based on a Surrogate Model Assisted Evolutionary Algorithm

Local optimization is a routine approach for full-wave optimization of microwave filters. For filter optimization problems with numerous local optima or where the initial design is not near to the optimal region, the success rate of the routine method may not be high. Traditional global optimization techniques have a high success rate for such problems, but are often prohibitively computationally expensive considering the cost of full-wave electromagnetic simulations. To address the above challenge, a new method, called surrogate model-assisted evolutionary algorithm for filter optimization (SMEAFO), is proposed. In SMEAFO, considering the characteristics of filter design landscapes, Gaussian process surrogate modeling, differential evolution operators, and Gaussian local search are organized in a particular way to balance the exploration ability and the surrogate model quality, so as to obtain high-quality results in an efficient manner. The performance of SMEAFO is demonstrated by two real-world design cases (a waveguide filter and a microstrip filter), which do not appear to be solvable by popular local optimization techniques. Experiments show that SMEAFO obtains high-quality designs comparable with global optimization techniques but within a reasonable amount of time. Moreover, SMEAFO is not restricted by certain types of filters or responses. The SMEAFO-based filter design optimization tool can be downloaded from http://fde.cadescenter.com